Low temperature continuous circulation reactor for the aqueous synthesis of ZnO films, nanostructures, and bulk single crystals

ABSTRACT

A method for synthesizing ZnO, comprising continuously circulating a growth solution that is saturated with ZnO between a warmer deposition zone, which contains a substrate or seed, and a cooler dissolution zone, which is contains ZnO source material.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation under 35 U.S.C. §120 of co-pendingand commonly-assigned U.S. Utility patent application Ser. No.12/761,246, filed on Apr. 15, 2010, by Jacob J. Richardson and FrederickF. Lange, entitled “LOW TEMPERATURE CONTINUOUS CIRCULATION REACTOR FORTHE AQUEOUS SYNTHESIS OF ZnO FILMS, NANOSTRUCTURES, AND BULK SINGLECRYSTALS,” which application claims the benefit under 35 U.S.C. Section119(e) of co-pending and commonly assigned U.S. Provisional PatentApplication Ser. No. 61/169,633, filed on Apr. 15, 2009, by Jacob J.Richardson and Frederick F. Lange, entitled “LOW TEMPERATURE CONTINUOUSCIRCULATION REACTOR FOR THE AQUEOUS SYNTHESIS OF ZnO FILMS,NANOSTRUCTURES, AND BULK SINGLE CRYSTALS,”;

all of which applications are incorporated by reference herein.

This application is related to the following co-pending andcommonly-assigned U.S. patent applications:

U.S. Provisional Application Ser. No. 61/257,811, filed on Nov. 3, 2009,by Jacob J. Richardson, Daniel B. Thompson, Ingrid Koslow, Jun Seok Ha,Frederick F. Lange, Steven P. DenBaars, and Shuji Nakamura entitled “ALIGHT EMITTING DIODE STRUCTURE UTILIZING ZINC OXIDE NANOROD ARRAYS ONONE OR MORE SURFACES, AND A LOW COST METHOD OF PRODUCING SUCH ZINC OXIDENANOROD ARRAYS,”;

U.S. Provisional Application Ser. No. 61/257,812, filed on Nov. 3, 2009,by Daniel B. Thompson, Jacob J. Richardson, Ingrid Koslow, Jun Seok Ha,Frederick F. Lange, and Steven P. DenBaars, and Shuji Nakamura, entitled“HIGH BRIGHTNESS LIGHT EMITTING DIODE COVERED BY ZINC OXIDE LAYERS ONMULTIPLE SURFACES GROWN IN LOW TEMPERATURE AQUEOUS SOLUTION,”; and

U.S. Provisional Application Ser. No. 61/257,814, filed on Nov. 3, 2009,by Daniel B. Thompson, Jacob J. Richardson, Steven P. DenBaars,Frederick F. Lange, and Jin Hyeok Kim, entitled “LIGHT EMITTING DIODESWITH ZINC OXIDE CURRENT SPREADING AND LIGHT EXTRACTION LAYERS DEPOSITEDFROM LOW TEMPERATURE AQUEOUS SOLUTION,”;

which applications are incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to ZnO, and a method and apparatus forfabricating the same.

2. Description of the Related Art

(Note: This application references a number of different publications asindicated throughout the specification by one or more reference numberswithin brackets, e.g., Ref [x]. A list of these different publicationsordered according to these reference numbers can be found below in thesection entitled “References.” Each of these publications isincorporated by reference herein.)

There has recently been a surge of interest in zinc oxide (ZnO); but, infact, the material has long been of importance for a wide variety ofapplications, from sunscreen and pigments, to rubber manufacturing andvaristors. However, these historic applications of ZnO have usually onlyrequired powdered forms of ZnO, or the polycrystalline ceramics formedfrom consolidating and sintering ZnO powders. As such, the ZnOrequirements of these applications have largely been met by thematerials produced directly from Zn ores or metallic Zn in large scale,“French” or “American” type industrial processes. Much of the recentinterest in ZnO stems from newly developed and yet to be developedpotential applications in the electronics and renewable energyindustries. Many of these emerging applications for ZnO will have morerigorous specifications on the form and quality of the ZnO used thantypical historical applications. Rather than simple powder, many ofthese applications will require ZnO in the form of thin films,nanoparticles, single crystals, and epitaxial material. Currentcommercial methods for producing these advanced forms of ZnO oftenutilize high temperatures, high or low pressures, toxic and/or highlyspecialized chemicals, and complex equipment, all of which lead to ahigh cost of production. In addition, each of the different forms of ZnOused for advanced applications typically requires its own specializedmethod for production.

Bulk ZnO single crystals are typically produced using either thehydrothermal method or a melt based growth method. The melting point ofZnO is near 2000° C., but ZnO at these temperatures will decompose to Znmetal and oxygen at atmospheric pressure. Therefore, melt based methodsfor producing ZnO crystals require extremely high temperatures, as wellas controlled atmosphere and/or pressure. Although the conditions usedare less extreme, the hydrothermal method still requires heavy-dutyautoclaves capable of withstanding the high temperatures (300-400° C.)and high pressures (80-100 MPa) used. The hydrothermal growth solutionsare also extremely corrosive and these autoclaves must be lined withnon-reactive materials like platinum. Compared to these methods, the lowtemperatures and atmospheric pressure used the current invention allowfor less energy consumption, less expensive equipment, and a lesshazardous process.

Industrially, ZnO thin films are often deposited by a physical vapordeposition method, like magnetron sputtering or pulsed laser deposition(PLD), but chemical vapor and chemical solution methods have also beenexplored. The major drawback of magnetron sputtering and PLD is the needto maintain the very low pressure growth environment needed to createthe plasma which sputters material from the target. The creation andcontrol of the plasma also require expensive equipment and significantamounts of power. Chemical vapor deposition techniques, such asmetalorganic chemical vapor deposition (MOCVD) also require low pressureatmospheres and expensive equipment, as well as specialty gases andchemical precusors. The present invention is related to the aqueoussolution deposition techniques of chemical bath deposition (CBD) andelectrodeposition, which can also be used to produce ZnO films, butoffers important advantages over these techniques. Like the currentinvention, both CBD and electrodeposition techniques produce ZnO fromdissolved Zn complexes. However, electrodeposition is limited toconductive substrates. The CBD has more versatility for substrates, buthas not been demonstrated for the deposition of epitaxial films.Typically, CBD also results in considerable amounts of precursors beingwasted.

The current industrial uses for ZnO nanostructures and nanoparticles arefairly limited. However as utilization increases, the current method haspotential to be more scalable than the other techniques for producingnanostructures and nanoparticles which are found in the academicliterature on the subject. Methods found in the literature for producingZnO nanostructures and particles include vapor techniques and bothaqueous and non-aqueous solution techniques.

The current invention presents a low temperature aqueous method thatcould potentially be used to synthesize all of the forms of ZnOmentioned above. While this technique has aspects in common with theestablished aqueous solution techniques of hydrothermal crystal growthand chemical bath deposition, crucial aspects of the current inventiongive the method disclosed here important advantages over these prior artaqueous solution methods, as well as over non-solution based techniques.

SUMMARY OF THE INVENTION

The method of the present invention utilizes a unique continuouscirculation method and reactor which allows for control over both thenucleation and growth mechanisms needed to synthesize ZnO films,nanostructures, and bulk single crystals. The method of the inventionuses an aqueous growth solution composition to dissolve a zinccontaining nutrient at a first temperature and synthesize ZnO at asecond temperature, where the second temperature at which ZnO issynthesized is warmer than the first temperature at which the nutrientwas dissolved, and wherein the synthesis of ZnO is caused by a reductionin the solubility of ZnO in the aqueous solution at the second, warmer,temperature compared to the first, cooler, temperature. In general, thereactor of the current invention consists of two distinct, butphysically connected zones containing an aqueous growth solution.Control over ZnO nucleation and growth is achieved by varying thechemical driving force for ZnO synthesis from the aqueous growthsolution using a combination of the solution composition, thetemperatures of the two reactor zones, and circulation of the solutionbetween the two zones. In the typical embodiment, the entire process isperformed at temperatures below the boiling point of the growth solutionand near ambient pressures.

The mild conditions possible with the disclosed process present severaladvantages over other methods. These advantages can include lower energyinput, lower equipment cost, and better compatibility with temperaturesensitive substrates, templates, or devices. Unlike other techniquescapable of depositing ZnO from aqueous solution at low temperature, suchas chemical bath deposition (CBD) or electrodeposition, which typicallyuse zinc salts as the source of Zn, one embodiment of the currentinvention uses ZnO itself. As a common industrial material, ZnO powderis inexpensive and readily available. Using ZnO as the Zn source alsoallows the reactor to operate in a closed loop, recrystallizing thedissolved ZnO source material into ZnO of the desired form, butrecycling all the other components of the growth solution. This meansthat the process produces no waste or by-products, further lowering theenvironmental impact and cost of the present method compared to existingmethods.

The two prior art methods for synthesizing ZnO most closely related tothe current invention are the hydrothermal growth method and chemicalbath deposition (CBD). The hydrothermal growth method of producing bulkZnO crystals also functions by recrystallizing ZnO powder, but thatmethod operates at significantly higher temperatures and pressures thanthe present invention. The temperatures and pressures used in thehydrothermal method, which are often above the supercritical point ofwater, are necessary to achieve the levels ZnO solubility and masstransport required for appreciable crystal growth rates in that method.In hydrothermal growth of ZnO, the chemical reactions controlling thedissolution and recrystallization of ZnO place can be written as:ZnO(cr)+(x−1)H₂O(l)

Zn(OH)_(x) ^(2-x)(aq)+(x−2)H⁺(aq),where x=1, 2, 3 or 4. Under the highly alkaline conditions used inhydrothermal growth of ZnO, higher temperatures shift the equilibrium ofthis reaction to the right, and lower temperatures shift it to the left.In other words, the solubility of ZnO increases with temperature, andtherefore, more ZnO can be dissolved in aqueous solution at highertemperatures than at lower temperatures. A typical hydrothermal growthprocedure takes advantage of this solubility dependence to grow bulkcrystals by dissolving ZnO powder, sometimes called the nutrient, in ahigher temperature zone, and then recrystallizing ZnO as a bulk singlecrystal in a cooler zone. The overall reaction can then be written asbelow:

In the hydrothermal method, transport of the dissolved zinc species fromthe hotter to the cooler zone takes place by diffusion and convectiveflow in the solution.

Like the hydrothermal method, the current invention utilizes atemperature variation in ZnO solubility to dissolve and thenrecrystallize ZnO. However, for the aqueous solutions used in the newmethod, the solubility dependence is reversed. This behavior is due tothe formation of soluble zinc complexes which are more stable at lowerthan higher temperatures. In one embodiment of the invention, thecomplexes used are zinc ammine complexes. Under some pH and temperatureconditions, zinc ammine complexes can form through the reaction ofdissolved ammonia with dissolved zinc ions or solid ZnO. In analogy tothe reaction written above for the hydrothermal method, ZnO solubilityin the present invention may be controlled by the reaction:ZnO(cr)+xNH₃(aq)+2H⁺(aq)

Zn(NH₃)_(x) ²⁺(aq)+H₂O(l),where x=1, 2, 3 or 4. Under the correct conditions of solution pH,ammonia concentration, and temperature, the equilibrium of this reactionwill shift to the left with increasing temperature and to the right withdecreasing temperature. This relationship is predicted by thethermodynamic calculations disclosed in Ref. [1].

Zinc ammine complexes are also utilized in CBD of ZnO, but in thattechnique, the reaction written above only moves from right to left.Like the current invention, CDB of ZnO tends to operate under relativelymild conditions, i.e., temperatures between room temperature and 100°C., atmospheric pressure, and solution pH that is neither extremelyacidic nor basic. When utilized in CBD, zinc ammine complexes are formedby reacting ammonia with the dissolved zinc ions supplied by a solublezinc salt, such as zinc nitrate, zinc chloride, zinc acetate, etc. Theformation of Zn complexes is used to control or moderate the depositionof ZnO, which is typically initiated by an increase in the temperatureor pH of the growth solution.

In the current invention, the zinc ammine complexes essentially functionas a carrier for transporting the dissolved zinc between the differentzones of the reactor. In the cooler zone of the reactor, where the ZnOnutrient is contained, the above reaction moves from left to right sothat zinc ammine complexes are formed by dissolving ZnO. In the hotterzone of the reactor, ZnO is synthesized by the reaction moving right toleft. When the growth solution is allowed to flow, or is otherwisecirculated, between the cooler and hotter zones, the overall reaction isas written below:

This overall reaction has been demonstrated to result in controllableZnO synthesis by the embodiment of the invention disclosed in Ref [2].

As described herein, ZnO has numerous applications requiring epitaxialand polycrystalline thin films, nanostructures, and bulk singlecrystals. The low temperature aqueous continuous circulation method andreactor which are the subject of this disclosure present a new low cost,environmentally friendly way of supplying the ZnO for theseapplications. Thus, to overcome the limitations in the prior artdescribed above, and to overcome other limitations that will becomeapparent upon reading and understanding the present specification, thepresent invention discloses a method for fabricating ZnO films,nanostructures, and bulk single crystals.

The method comprises (a) using an aqueous growth solution composition todissolve a zinc containing nutrient at a first temperature; and (b)synthesizing the ZnO at a second temperature, wherein the secondtemperature at which the ZnO is synthesized is higher or warmer than thefirst temperature at which the nutrient is dissolved, and the synthesisof ZnO is caused by a reduction in solubility of the ZnO in the aqueoussolution at the second, warmer, temperature compared to the first,cooler, temperature.

The method may further comprise moving, or allowing to move, the aqueoussolution composition between at least two zones including a first zoneat the first, cooler, temperature containing the nutrient, and a secondzone at the second, warmer, temperature where ZnO is synthesized, sothat a net flux of ZnO occurs from the first zone to the second zone.

The method may further comprise one or more process steps includingchanging the respective temperatures of the first zone or the secondzone, or the first zone and the second zone, by heating or cooling, orheating and cooling the first zone or the second zone.

A rate of the ZnO synthesis may be controlled by varying any combinationof the aqueous solution's composition, the first temperature of thefirst zone and the second temperature of the second zone, heating orcooling rates, or the heating and the cooling rates, of the first zoneand the second zone, and a rate at which the aqueous solutioncomposition moves between the first zone and the second zone.

A nucleation rate of the ZnO, either on a substrate or template or asfree nuclei, may be controlled by varying any combination of the aqueoussolution's composition, the first temperature of the first zone and thesecond temperature of the second zone, a heating or a cooling rate, orthe heating and the cooling rates, of the first zone and the secondzone, and a rate at which the aqueous solution composition moves betweenthe first zone and the second zone.

Both the first, cooler, temperature, at which the nutrient is dissolved,and the second, warmer, temperature, at which the ZnO is synthesized,may be between a freezing point and a boiling point of the aqueoussolution composition.

The zinc containing nutrient may be ZnO. The nutrient may be ZnO thathas been doped or alloyed with another substance. The aqueous solutioncomposition may contain complexing ligands, have an appropriate pH, andotherwise comprise a composition where zinc complexes form, resulting ina higher solubility at the first, cooler, temperature compared to thesecond, warmer, temperature. The complexing ligand used may be ammoniaso that zinc ammine complexes form in the aqueous solution composition.

The aqueous solution composition in the second zone may be heated toobtain the second temperature in the second zone that is warmer than thefirst temperature in the first zone. The aqueous solution composition inthe first zone may be cooled to obtain the first temperature in thefirst zone which is cooler than the second temperature in the secondzone. The aqueous solution composition in the first zone may be cooledand the aqueous solution composition in the second zone may be heated toobtain the second temperature in the second zone that is warmer than thefirst temperature in the first zone.

During one or more process steps, the aqueous solution composition maybe prevented from moving between the first zone and the second zone.

The ZnO that is synthesized may be doped or alloyed with one or morecomponents.

The ZnO synthesis may result in formation of a ZnO film on a substratein contact with the aqueous solution composition. The substrate may havealready been seeded with ZnO prior the synthesis of the ZnO. Theresulting ZnO film may be epitaxial with the substrate. The substratemay be a Group III-Nitride based light emitting diode device. The ZnOsynthesis may result in formation of ZnO microstructures ornanostructures and the microstructures or the nanostructures form eitheron, or within, a substrate template, or in a bulk of the aqueoussolution composition. The ZnO synthesis may result in growth of a bulkZnO single crystal.

The aqueous solution composition may contain additives which affect thesynthesis of the ZnO but are not consumed during the ZnO synthesis andare not incorporated into the resulting ZnO that is synthesized, and theadditives may include (but are not limited to) one or more of thefollowing: metal citrate salts, citric acid, other salts or acids whichproduce stable anions that interact with the surfaces of the ZnO,surfactants, polymers, and/or biomolecules.

The aqueous solution composition may contain dissolved species which areincorporated into the ZnO that is synthesized. The dissolved species mayinclude, but are not limited to, ions containing elements which areincorporated into the ZnO as dopants.

The current invention further comprises a continuous circulation reactorfor fabricating ZnO, comprising a dissolution zone or vessel, fordissolving a ZnO nutrient into an aqueous growth solution; a depositionzone or vessel, for depositing ZnO from the aqueous growth solution ontoa substrate or seed; and a connection to the dissolution zone such thatmovement of the aqueous growth solution from the dissolution zone to thedeposition zone, and vice versa, occurs.

The dissolution zone and the deposition zone may be for operation at oneor more temperatures between a freezing point and a boiling point of theaqueous growth solution. The dissolution zone and the deposition zonemay be contained in one or more vessels which are fabricated fromnonreactive materials that resist corrosion, dissolution, or otherdegradation by contact with the aqueous growth solution. The nonreactivematerials include, but are not limited to, one or more of the following:fluoropolymers, other higher performance polymers, silicate glass,and/or stainless steel. All components of the reactor which come intocontact with the aqueous growth solution during operation may becomposed of the nonreactive materials.

The reactor may further comprise a peristaltic pump, tube pump, or othermechanical pumping mechanism capable operating in a manner that does notcontaminate the aqueous growth solution being pumped, positioned to pumpthe aqueous aqueous growth solution from the dissolution zone to thedeposition zone, and capable of controlling a circulation rate of theaqueous growth solution between the dissolution zone and the depositionzone.

The reactor may further comprise a filter positioned between thedissolution zone and the deposition zone to prevent any, or limit theparticle size of, undissolved nutrient being pumped from the dissolutionzone to the deposition zone.

The reactor may further comprise an overflow mechanism positioned toreturn flow of the aqueous growth solution from the deposition zone tothe dissolution zone, and for ensuring that a volume of the aqueousgrowth solution in each zone remains constant while avoiding the needfor a second synchronized pump to return the aqueous growth solution tothe dissolution zone.

The reactor may further comprise a heater for heating the depositionzone; and a temperature sensor in thermal contact with the depositionzone, wherein the heater is for heating the growth solution in thedeposition zone, such that the growth solution in the deposition zone isa warmer growth solution and the aqueous growth solution in thedissolution zone is a cooler growth solution that is cooler than thewarmer growth solution, and the temperature sensor enables measurementand control of a temperature in the deposition zone through anelectronic temperature controller or computer connected to both theheater and the temperature sensor. The aqueous growth solution in thedeposition zone of the reactor may be heated by absorption of radiationrather than by thermal contact with a heater, and in which case theheater is the radiation or a source of the radiation. A substrate orseed crystal in the deposition zone of the reactor may be first heatedby the heater and heat then transferred from the substrate or seed tothe surrounding aqueous growth solution.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring now to the drawings in which like reference numbers representcorresponding parts throughout:

FIG. 1 is a flowchart and cross-sectional schematic illustrating amethod of, and reactor for, fabricating, synthesizing, or fabricatingZnO according to the present invention.

FIGS. 2( a) and 2(b) show the calculated solubility ZnO in aqueoussolution as a function of pH and ammonia concentration at 25° C. and 90°C., respectively, wherein the highlighted areas draw attention to arange of pH and ammonia concentration where the solubility of ZnO islower at 90° C. than 25° C.

FIG. 3 is a cross-sectional schematic of an apparatus for fabricatingZnO according to the present invention, modified from Ref. [2].

FIGS. 4( a)-(c) show scanning electron microscope images of epitaxialZnO on MgAl₂O₄ substrates produced by microwave heating of an aqueousgrowth solution.

FIG. 5( a) and FIG. 5( b) show cross-sectional schematic of GroupIII-Nitride (III-N) Light Emitting Diode (LED) devices utilizingepitaxial ZnO current spreading layers fabricated using the currentinvention, and FIG. 5( c) and FIG. 5( d) show optical images of III-NLED devices utilizing epitaxial ZnO current spreading layers fabricatedusing the current invention emitting blue light during operation.

DETAILED DESCRIPTION OF THE INVENTION

In the following description of the preferred embodiment, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration a specific embodiment in which theinvention may be practiced. It is to be understood that otherembodiments may be utilized and structural changes may be made withoutdeparting from the scope of the present invention.

Overview

A novel process is presented for the synthesis of ZnO from lowtemperature aqueous solution using a continuous circulation method andreactor. By utilizing the temperature dependence of ZnO solubility inammoniacal aqueous solution, between the freezing and boiling range ofthe solution, to drive a dissolution and recrystallization process, themethod may be used to transform a Zn source, such as ZnO powder, intoZnO of another desired form, which could be a film, nanostructures, or abulk single crystal. The reactor for performing this method is comprisedof at least two temperature zones including a cooler zone in which theZn source is dissolved, and a warmer zone where the ZnO product isformed. During operation, the aqueous growth solution may be circulatedbetween these two zones. This circulation process allows soluble zinccomplexes acting as zinc carriers, to transport zinc from the coolerzone, where the complexes are formed, to the warmer zone, where thecomplexes dissociate and ZnO is recrystallized. The nucleation andgrowth of ZnO in the reactor is controlled though a combination of thesolution composition, the circulation of growth solution between thereactor zones, and the temperature management of the reactor zones. Bycontrolling the nucleation and growth processes and supplying theappropriate substrate or seed crystal, this basic reactor design can beused to synthesize ZnO powders, nanoparticles, polycrystalline films,epitaxial films, and bulk single crystals. The reactor can use verysimple, relatively non-toxic, inexpensive chemicals, and because itoperates in a closed cycle, it produces little to no waste byproducts.

Technical Description

ZnO Synthesis Using a Continuous Circulation Reactor Method

FIG. 1 is a flowchart and schematic illustrating an embodiment of amethod for, and reactor for, fabricating, synthesizing, growing, ordepositing ZnO according to the present invention, comprising moving(e.g., continuously circulating 100), or allowing the movement of, agrowth solution containing dissolved Zn species between a cooler (cold)dissolution vessel or zone 102, which contains ZnO nutrient material,and a warmer (hot) deposition vessel or zone 104, which may contain asubstrate, seed crystal, or template, wherein the solubility of the ZnOin the growth solution is lower in the warmer deposition zone 104 thanin the cooler dissolution zone 102, so that ZnO nutrient is dissolved inthe cooler dissolution zone 102 and ZnO of the desired form isfabricated, synthesized, grown, or deposited in the warmer depositionzone 104. A cooler 102 and a warmer 104 zone within a single vessel canalso be used. The circulating or moving 100 comprises cold solution flow106 and hot solution flow 108. The warmer zone 104 is warmer or hotterthan the colder or cooler zone 102. In one embodiment of the invention,the temperatures of the cooler zone 102 and warmer zone 104, as well asthe circulation of solution between them, may all be variedindependently.

Steady-state operation is obtained if the volumes and temperatures ofthe solutions within both the deposition zone 104 and the dissolutionzone 102 are maintained at constant levels while growth solution iscontinuously circulated 100. During steady-state operation, the coolersolution heats 110 after entering the warmer vessel or zone 104. Theheating of the entering solution causes equilibrium solubility of ZnO inthat solution to lower or decrease 112. If the equilibrium solubility islowered or decreased below the concentration of Zn in solution, theresult is a supersaturation 114 of dissolved ZnO in the solution. Toreturn the Zn concentration in solution towards equilibrium, ZnOformation 116 occurs. ZnO formation 116 may occur through the synthesisof new ZnO nuclei or the growth of existing ZnO crystals. As the warmsolution then returns 108 to the cooler vessel 102, its temperaturedecreases (hot solution cools 118), raising or increasing the solubility120 of ZnO in solution. This causes the solution to becomeunder-saturated (undersaturation 122), so that ZnO source material ornutrient is now dissolved (ZnO nutrient dissolution 124) to return thesolution towards equilibrium. This cycle can continue uninterrupteduntil all of the ZnO nutrient in the cooler dissolution zone 102 hasbeen dissolved.

The overall effect of the above solution moving, or circulating, processis a net ZnO flux or flow 126 from the cooler dissolution zone 102 tothe warmer deposition zone 104. If the solution in both the coolerdissolution zone and warmer deposition zone remains near equilibrium,the flux 126 will be equal to the rate of ZnO supersaturation in thedeposition zone 104, and can be expressed with the following equation:

${Flux} = {\frac{\partial\sigma}{\partial t} = {r_{circ}\left( {{C_{ZnO}\left( T_{cool} \right)} - {C_{ZnO}\left( T_{hot} \right)}} \right)}}$

Here, t is time, σ is the supersaturation, r_(circ) is the rate ofsolution circulation, and C_(ZnO) is the equilibrium solubility of ZnOexpressed at T_(cool), the temperature of the cooler dissolution zone102, and at T_(hot), the temperature of the warmer deposition zone 104.However, even without solution circulation 100, initial heating of thesolution in the deposition zone 104 can also result in ZnO formation116. In the case of initial heating without circulation, there will beno resulting ZnO flux 126, but the rate of ZnO supersaturation in thedeposition zone 104 can be expressed with the following equation.

$\frac{\partial\sigma}{\partial t} = {{- \frac{\partial C_{ZnO}}{\partial T_{hot}}}\frac{\partial T_{hot}}{\partial t}V_{hot}}$The new variable, V_(hot), is the volume of solution heated in thedeposition zone 104. If the solution remains near equilibrium duringheating, the rate of ZnO formation can be approximated by the equationfor the rate of supersaturation. This rate of ZnO formation will varywith time and is therefore not steady-state. If the reactor is operatingwith circulation and variable temperatures, the two equations for therate of ZnO formation in the deposition zone 104 are simply summedgiving the new supersaturation equation below, which again approximatesthe rate of ZnO synthesis 116 in the deposition zone 104.

$\frac{\partial\sigma}{\partial t} = {{{- \frac{\partial C_{ZnO}}{\partial T_{hot}}}\frac{\partial T_{hot}}{\partial t}V_{hot}} + {r_{circ}\left( {{C_{ZnO}\left( T_{cool} \right)} - {C_{ZnO}\left( T_{hot} \right)}} \right)}}$

Since the ZnO flux 126 occurs via dissolution and recrystallization, theprocess can transform any arbitrary form of ZnO into another. In thepreferred embodiment, low cost ZnO powder is dissolved in the coolerdissolution zone 102 and higher value nanostructures, films, or bulksingle crystals are formed 116 in the hotter deposition zone 104. It isalso possible to use other zinc containing substances as the Zn nutrientin the dissolution zone 102. Possibilities include zinc containingsubstances with partial solubility in aqueous solution like Zn(OH)₂,hydrated zinc oxide or zinc hydroxide, zinc citrate, etc. However, onlyZnO can be used with the reactor operating in a closed loop, as shown inFIG. 1, without changing the solution composition over time. Withanother nutrient besides ZnO, the equations as written above forsupersaturation rate must be modified.

Solution Composition

The correct solution composition is critical for the continuouscirculation reactor to operate according to the mechanism illustrated inFIG. 1. The two most important aspects of the solution composition forthe present invention are the pH of the solution and the concentrationof appropriate complexing ligands. With an aqueous solution which doesnot contain the appropriate complexing ligands, ZnO will tend to be moresoluble at higher temperatures than lower, and the reactor design of thepresent invention would not synthesize ZnO. It is therefore necessary toprovide a source of ligands to form zinc complexes which act to increasethe solubility of ZnO at lower temperatures more than at higher ones.

A working embodiment of the invention uses a solution containingdissolved ammonia (NH₃) to provide ammine ligands for this task, but itis intended that current invention extend to the use of other sources ofammine ligands, as well as other ligands which result in a similar ZnOsolubility dependence on temperature. Ammine is the name given to theNH₃ molecular unit when it is functioning as a ligand in a complex.Aqueous solutions of ammonia dissolved in water are also commonlyreferred to as an ammonium hydroxide (NH₄OH) solution, ammonia water,aqua ammonia, household ammonia, or simply ammonia. Ammonia ligands canalso be supplied to an aqueous solution by dissolving ammonium salts.Examples include, but are not limited to, simple inorganic and organicsalts such as ammonium chloride (NH₄Cl), ammonium nitrate (NH₄NO₃),ammonium acetate (CH₃COONH₄), ammonium carbonate ((NH₄)₂CO₃),Triammonium citrate ((NH₄)₃C₆H₅O₇), etc. Ammonia ligands could also besupplied as part of a soluble coordination compound or double salt.Additionally, ammonia ligands could be supplied by the en situdecomposition of another compound, urea or hexamine for example. Otherligands besides ammine which form aqueous complexes of Zn(II) and whichresult in a temperature range of decreasing solubility of ZnO withincreasing temperature may also be used. Other ligands likely to behavein this manner include, but are not limited to, water soluble primaryamines, secondary amines, tertiary amines, and polyamines. Amines areammonia based organic compounds where at least one hydrogen is replacedwith an alkyl or aryl group. Non-nitrogen containing ligands which formcomplexes which result in the required solubility behavior for ZnO canalso be used.

The thermodynamic calculations disclosed in Ref [1] predicted thataqueous solutions containing ammonia can have substantially highersolubility for ZnO at room temperature (25° C.) than at near boilingtemperatures (90° C.). FIG. 2, which is modified from Ref. [1], showsthe results of ZnO solubility calculation made as a function of pH,ammonia concentration, and temperature. FIG. 2( a) shows the calculatedsolubility at a temperature of 25° C. and FIG. 2( b) shows the same at90° C. The regions 200 and 202 highlight the same example pH and ammoniaconcentration range in FIG. 2( a) and FIG. 2( b), respectively, andclearly show that ZnO solubility is significantly lower for the 90° C.calculation area 202.

The experimental results disclosed in Ref. [2] demonstrated that anembodiment of the current invention can deposit ZnO from solutionscontaining between 0.25 and 1.0 mol/L ammonia and having pH between 10and 12. These conditions are understood to be only examples of the manypH and ammonia concentration solution conditions capable of producingZnO with the present invention and do not represent any sort offundamental or practical limit on the possible conditions. However, boththe calculation in Ref [1] and the experimental results in Ref. [2] domake it clear that a solution that does not contain ammonia, or anothersource of appropriate complexing ligands, will not produce ZnO using themethod of this invention.

In a simplest embodiment of the invention, the aqueous growth solutioncontains only the dissolved zinc source, ammonia or another source ofsimilarly acting complexing ligands, and whatever acid or base isnecessary to achieve the desired pH. To avoid making the ZnO solubilitybehavior more complicated, only acids and bases which do not formcomplexes with zinc under the pH and temperature conditions utilizedshould be used. In addition to these simplest compositions, it is alsopossible to use more complex growth solutions containing additives tomodify ZnO growth or composition. This could include additives such ascitrate ions, which are well known to affect the morphology of ZnOsynthesized in aqueous solution. For example, citrate ions can beutilized in the growth solution though the addition of soluble metalcitrate salts or citric acid. Citric acid is thought to preferentiallyadsorb to certain crystallographic surfaces of ZnO during growth andthereby act to slow growth on those surfaces. Because the citrate onlyadsorbs to the surface of ZnO, but does not become incorporated, thecitrate ions in solution are conserved and recycled. Examples of otheradditives likely to show similar behavior include other poly-anionicmolecules, surfactants, water soluble polymers, and biomolecules.

Additives can also be made to the growth solution with the intention ofchanging the composition of the ZnO synthesized. Additions of this typecould include sources of group III elements such as Al, Ga, or In, whichare known to n-type dope ZnO, Group I elements such as Li, which isknown reduce the conductivity of ZnO, or isovalent elements, like Mg orCd, which are known to modify the bandgap of ZnO when used as dopants.The addition of dopant additives such as those mentioned above could beachieved by completely dissolving a highly soluble dopant containingchemical to the growth solution. For example, Al could be supplied bythe complete dissolution into solution of Al nitrate. The addition ofdopant to the solution could also be achieved in the same manner as thezinc, i.e., dissolving a source of the dopant and maintaining theconcentration of dopant with an excess of the dopant source in thedissolution zone along with the zinc nutrient. For example, Al₂O₃ powdercould be mixed with the ZnO powder in the dissolution zone. A thirdmethod would be to dope the Zn nutrient before using it in the reactor,e.g., using an Al doped ZnO powder as the nutrient. The second and thirdmethods have the advantage of maintaining the concentration of dopant insolution throughout the growth. However, if only a small amount ofdopant is being incorporated into the ZnO relative to the solutionconcentration, the solution concentration will change very little, andthe first method may work just as well.

Controlling Nucleation and Growth of ZnO

According to classical nucleation theory, the free energy change AGrelated to the creation of a nucleus of one phase in another can beexpressed as the sum of a volumetric energy term, which goes as theradius, r, of the nucleus cubed, and a surface energy term, which goesas the radius squared:

${\Delta\; G} = {{\frac{4}{3}\pi\; r^{3}G_{v}} + {4\;\pi\; r^{2}\gamma}}$The volume term is proportional to the volumetric chemical free energy,G_(v). When the chemical reaction leading to the new phase isenergetically favorable, G_(v) is negative, when the reverse reaction isfavorable, G_(v) is positive. The further the reaction from equilibrium,the greater the magnitude of G_(v), and when the reaction is in chemicalequilibrium, G_(v) is zero. A nucleation process implies that newsurface is created, so the surface energy term is always positive andproportional to the surface energy, γ. Because of the dependence on thesquare of the nucleus radius compared to the cube dependence of thevolume term, the surface term will always dominate for small radii andthe volume term will dominate at large radii. At a critical radii givenby,

${r^{*} = {- \frac{2\;\gamma}{G_{v}}}},$ΔG has a maximum given by,

${\Delta\; G^{*}} = {\frac{16\;\pi\; y^{3}}{9\; G_{v}^{2}}.}$The quantity ΔG* represents the energy barrier to the formation of astable nucleus, which controls the nucleation rate, I, through anArrhenius type relation given by,

$I \propto {{\mathbb{e}}^{\frac{{- \Delta}\; G^{*}}{k_{B}T}}.}$Although these equations are specific to homogeneous nucleation, thebehavior for other forms of nucleation can be expressed similarly. Inthe solution phase, non-homogeneous nucleation will always results in alower value for ΔG*, and thus, a higher nucleation rate. From least tomost thermodynamically favorable, the types of nucleation possible inthis system are, homogeneous, heterogeneous, heteroepitaxial, andhomoepitaxial (growth).

As discussed above, heating the growth solutions used in the presentinvention lowers the solubility of ZnO in solution. Unless ZnO isprecipitated or deposited from solution, a supersaturation condition iscreated. In the thermodynamic terms used above, a supersaturation meansthat the equilibrium has shifted towards the synthesis of ZnO so thatG_(v) is negative. The greater the supersaturation, the more negativeG_(v) is. The more negative G_(v) is, the lower the energy barrier tonucleation is, and thus, the greater the nucleation rate. However, if agrowth solution is heated relatively slowly, the magnitude of G_(v) thatcan actually occur is limited. This is due to the fact that once G_(v)reaches some critical value, nucleation of ZnO will start to occur at anappreciable rate. If a ZnO seed is present, homoepitaxial nucleation, orgrowth, will occur at very low supersaturations. Given a appropriatesubstrate, heteroepitaxial nucleation can occur but will require ahigher superstation. If the substrate present has no epitaxialrelationship with ZnO, heterogeneous nucleation will typically requirean even higher supersaturation. If no heterogeneous nucleation sites areavailable, homogeneous nucleation can occur but requires the highestsupersaturation. Once nucleation begins, growth of the nuclei allows thesystem to move back towards equilibrium, thereby reducing thesupersaturation and G_(v). As long as the maximum kinetically allowedgrowth rate of ZnO is faster than the rate of change of thesupersaturation, the solution will remain near equilibrium. The rate ofchange in supersaturation can be quantified in the supersaturation rateequation introduced above and rewritten here:

$\frac{\partial\sigma}{\partial t} = {{{- \frac{\partial C_{ZnO}}{\partial T_{hot}}}\frac{\partial T_{hot}}{\partial t}V_{hot}} + {r_{circ}\left( {{C_{ZnO}\left( T_{cool} \right)} - {C_{ZnO}\left( T_{hot} \right)}} \right)}}$Therefore, if the supersaturation rate is low, and the solution remainsnear equilibrium, the rate of ZnO synthesis in the deposition zone ofthe reactor will be equal to the supersaturation rate above. As aconsequence, at low supersaturation rates, the rate of ZnO synthesis canbe affected by the solution composition, circulation rate, the volume ofthe deposition zone, the temperature of the deposition and dissolutionzone, and the rate of temperature change of the deposition zone.

The description above can provide guidelines for selecting theconditions used to fabricate different forms of ZnO. For example, whengrowing a bulk single crystal, the supersaturation rate should be keptlow in order to prevent secondary nucleation and dendritic growth, whichoccurs at fast growth rates and lowers crystal quality. Forming anepitaxial film on a substrate may require a higher initialsupersaturation rate to initiate nucleation, followed by lowersupersaturation rate to facilitate high quality single crystal filmgrowth. The synthesis ZnO nanowires on a substrate could require a lowernucleation density than a film, so the wires stay separated, but ahigher supersaturation rate after nucleation to promote 1-dimensionalgrowth. Synthesis of free nanoparticles would require a very highsupersaturation rate to initiate the nucleation of many particles, butthe supersaturation would then need to immediately drop to preventfurther growth of the particles.

Mechanisms for Heating and Cooling the Growth Solution

Temperature control of the solution in the continuous circulationreactor can be achieved in numerous ways. In the simplest embodiment,only the temperature of the deposition zone is actively controlled, andthe temperature of the dissolution zone remains near the ambienttemperature. This still allows for several options for heating thedeposition zone. The continuous circulation reactor may use a heatingelement in thermal contact with deposition vessel to heat the enclosedsolution. This method has the simplest equipment requirements, but mayhave difficulty achieving very rapid or uniform heating rates, dependingon the size, shape, and composition of the deposition vessel. Since theheat originates from the walls of the vessel when using such externalheating, there is a finite limit to the rate heat can be transferred tothe solution. Even with mixing, the solution in contact with the wallwill heat first, increasing the chance of nucleating ZnO on the walls ofthe reactor vessel.

Another embodiment uses heating the solution though absorption ofradiation, for example, commonly used 2.45 GHz microwave radiation.Microwave radiation heats the solution directly through the dielectricheating effect and may allow for more uniform temperature distributionsover a wider range of heating rates. Microwave heating may be controlledby varying the power level of the microwave radiation or by variablypulsing a constant microwave power. An important feature of any heatingmethod to be used is an accurate feedback mechanism for controlling thetemperature. For microwave heating this still may be achieved using athermocouple to measure the temperature of the growth solution, as isdone in Ref. [2], but this may require screening the thermocouple fromthe microwave electric and magnetic fields, so other electronic oroptical temperature measuring schemes may be better suited.

Yet another embodiment uses direct heating of a substrate or seedcrystal which is in physical contact with the growth solution. In thisembodiment, the heater is in direct thermal contact with the substrateor another material in thermal contact with the substrate and not indirect thermal contact with the solution or the reactor vessel walls.Heat is transferred from the substrate or seed crystal to the adjacentgrowth solution. This flow of heat should cause synthesis of ZnO topreferentially occur in the immediate vicinity of the substrate or seed.For film or single crystal growth, this method may increase growth ratesand reduce the amount of ZnO formed away from the intended substrate orseed crystal.

Working Embodiments of a Continuous Circulation Reactor

FIG. 3 illustrates a working embodiment of an apparatus 300 comprising acontinuous circulation reactor. To minimize contaminates in the growthsolution 302 a, 302 b, the reactor is constructed entirely of chemicallyinert fluoropolymer, perfluoroalkoxy (PFA) (e.g., pre-molded PFAcomponents obtained from Savillex Corporation). To prevent evaporationof the growth solution, the reactor is sealed off from the surroundingatmosphere during operation. The reactor comprises of two vessels: afirst “cold” dissolution vessel 304, where the ZnO powder (Zn nutrient306) is dissolved into an aqueous solution 302 a, and a second “hot”deposition vessel 308, where ZnO is synthesized from the growth solution302 b. In this embodiment, the growth solution 302 a, 302 b is comprisedof an aqueous solution of ammonia also containing any nitric acid and/orsodium hydroxide added to adjust the pH to the desired level along withwhatever soluble zinc species are formed by allowing the solution todissolve ZnO by equilibrating with an excess of ZnO powder. In thisembodiment, the dissolution vessel 304 and the deposition vessel 308 aretypically (although not necessarily) for operation at one or moretemperatures below the boiling point of the growth solution and ambient(e.g., atmospheric) pressure. In this embodiment, the dissolution vessel304 corresponds to the dissolution zone 102 in FIG. 1 and the depositionvessel 308 corresponds to the deposition zone 104 in FIG. 1.

The reactor further comprises a mechanical fluid pump 310, positioned topump the solution 302 a from the first vessel 304 to the second vessel308 and to control the circulation rate of the solution 302 a, 302 bbetween the dissolution vessel 304 and the deposition vessel 308. Inthis embodiment, the pump 310 is a peristaltic pump so that the solutionwas only exposed to the inside of the Teflon vessel 304, 308 and thetubing 312. The circulation direction 314 is also shown. The solution302 a is pumped from the dissolution vessel 304 through an in-linefilter 316, comprised of a series of porous polytetrafluoroethylene(PTFE) membranes 316, in order to prevent any ZnO powder (nutrient) 304from being transferred to the deposition vessel 308. Another tubeconnects the deposition vessel 308 to the dissolution vessel 304 nearthe top of each vessel and acts as an overflow mechanism 320, so thatthe solution 302 b may flow from the deposition vessel 308 to thedissolution vessel 304 via the overflow mechanism 320. Thus, return flowof the growth solution from the deposition vessel 308 to the dissolutionvessel 304 may be accomplished by an overflow mechanism 320, ensuringthat the volume of growth solution 302 a, 302 b in each vessel 304, 308remains constant during operation.

The reactor further comprises a heater 322 in thermal contact with thedeposition vessel 308, which is then in thermal contact to the growthsolution 302 b. In this embodiment, the heater 322 was an electricalheating element in the form of a tape wrapped around the outside ofdeposition vessel 308. A temperature sensor 324 is in thermal contactwith the growth solution 302 b. In this embodiment, the temperaturesensor 324 was a PTFE coated type k thermocouple. Both the heating tape322 and the thermocouple 324 are electrically connected to an electronictemperature controller 326. The electronic controller is thus capable ofoperating a feedback loop between the signal from the thermocouple 324measuring the temperature of the deposition vessel solution 302 b andthe electrical heating tape 322 heating the deposition solution 302 b.This allows for the deposition solution 302 b to be heated at a constantrate and held at a constant temperature. To maintain a uniformconcentration and temperature profile in the solution 302 b within thedeposition vessel 308, the solution 302 b is continuously stirred usingPTFE coated magnetic “stir-bar” 328 coupled with a magnetic stir-plate330 positioned below the deposition vessel 308. In this embodiment, thedissolution vessel solution 302 a is cooled by heat transfer to theambient atmosphere via the walls of the dissolution vessel 304. Thismechanism of cooling only works when the rate of heat dissipation from304 to the ambient can match the rate that heat is added by the flow ofsolution into the dissolution vessel 304. This can be true when the rateof solution circulation is slow and/or the surface area of thedissolution vessel 304 is large.

For deposition on a substrate, the embodiment illustrated FIG. 3utilizes a sample holder 332, also made from PFA, which suspends thesubstrate in the relative center of the deposition vessel 308 leavingthe surface of the substrate completely exposed to the growth solution302 b. Various substrates can be used in the present embodiment. Thisincludes substrates which allow epitaxial ZnO formation such as (111)oriented single crystal MgAl₂O₄ wafers, (0001) oriented sapphire waferswith epitaxial (0001) oriented GaN buffer layers, single crystal GaNwafers with various orientations including, (0001), (10-10), (10-11),and III-N LED devices based on any of the preceding substrates.Epitaxial growth may also be performed on a single crystal ZnO wafer, oran other wise formed single crystal ZnO seed. Possible substrates alsoinclude materials which do not allow epitaxial ZnO formation, includingglasses and polymers based substrates.

The continuous circulation reactor embodiment illustrated in FIG. 3 anddescribed above was used to produce the experimental results disclosedin Ref. [2] for the deposition of epitaxial (0001) oriented ZnO onto(111) oriented single crystal MgAl₂O₄ substrates. Using this embodiment,epitaxial ZnO was successfully deposited using aqueous solutions with aroom temperature pH 11 having ammonia concentrations of 0.25, 0.5, and1.0 mol/L, and solutions with room temperature pH 10 and 12 having anammonia concentration of 0.5 mol/L. Further, the results in Ref. [2]demonstrate that the growth rate of ZnO can be controlled by thesolution composition (pH and ammonia concentration), the circulationrate, and the temperature and heating rate of the deposition vesselsolution.

Experimental Microwave Heating Results

Experimental results have also been obtained using microwave heating tofacilitate ZnO synthesis from the filtered solution compositionsutilized in the deposition zone of the present invention. Theseexperiments were performed without circulation with the dissolutionzone. The rapid heating possible using microwave radiation was shown toresult in a high density of epitaxial nuclei on (111) MgAl₂O₄ and GaNsubstrates and a rapid growth of a film. However, the density of nucleiand rate of growth were controlled by varying the conditions ofsolution, pH, ammonia concentration, and heating rate. Scanning electronmicroscope (SEM) images of ZnO synthesized using microwave heating areshown in FIG. 4( a) and FIG. 4( b).

FIG. 4( a) shows an 45° tilted cross-section SEM image of an epitaxialZnO film 400 deposited on a (111) oriented MgAl₂O₄ substrate 402 byrapidly heating a pH 11 solution containing 1 mol/L dissolved ammoniaand saturated with dissolved ZnO using 2.45 GHz microwave radiationsupplied by a Biotage Initiator® single mode microwave reactor. Thesolution was heated to 90° C. in approximately 40 seconds, and thenimmediately allowed to cool to approximately 50° C. before removing thesubstrate. Even in such a short growth period, the film has alreadygrown to a thickness of approximately 600 nm.

FIG. 4( b) shows a similarly formed epitaxial film where the growthsolution was held at 90° C. for 25 minutes for cooling to ˜50° C. andremoving the substrate. The addition time in solution at 90° C. hasallowed this film to grow to approximately 2 μm in thickness.

FIG. 4( c) shows how different conditions, in this case solution pH,result in epitaxial nanorods rather than a film. These ZnO nanorods weresynthesized on a (111) oriented MgAl₂O₄ by heating a ZnO saturated pH12, 0.5 mol/L ammonia solution. As before, the solution was heated to90° C. in approximately 40 seconds, and was then held at 90° C. for 1minute before cooling to ˜50° C. and removing the substrate.

III-N LED Current Spreading Layers Fabricated Using the PresentInvention

Epitaxial ZnO films fabricated using the present invention havedisplayed carrier concentrations greater than 10¹⁸ cm⁻³ and electronmobilities greater than 50 cm²/Vs, as measured by Hall effectmeasurements. Optical transmission measurements on epitaxial ZnO filmsproduced using the current invention have shown the films to transmitover 90% of the light in the visible spectrum. These values give filmsof ZnO a low enough sheet resistance and a high enough transparency tofunction as a transparent current spreading layer when deposited on aIII-N LED. The benefit of an epitaxial ZnO current spreading layerdeposited on a GaN based blue LED from low temperature aqueous solutionwas recently demonstrated by Thompson et al. in Ref [3], where such alayer was shown to improve the external quantum efficiency of a blue LEDby ˜93% compared to a NiO/Au current spreading layer.

FIG. 5( a) illustrates the architecture of the LED 500 fabricated in Ref[3], comprising a sapphire substrate 502, n-GaN layer 504 on thesubstrate 500, active layer 506 on the n-GaN layer 504, p-AlGaN layer onthe active layer 506, and p-GaN layer 508 on the AlGaN layer. Layers502-508 form the substrate upon which n-type ZnO 510 may begrown/deposited, in aqueous solution (e.g. at 90° C.) according to themethod of the present invention. The substrate (layers 502-508) may beplaced in (e.g., submerged in) growth solution, in the deposition vesselof the present invention's reactor, so that the n-ZnO 510 may be grownon layer 508. Also shown are the p-type contact pad 512 and n-typecontact pad 514.

FIG. 5( b) illustrates the similar architecture of a device fabricatedusing a single crystal GaN substrate 516. FIG. 5( c) shows an opticalimage of the device fabricated in Ref. [3] producing blue light undercurrent injection. FIG. 5( d) shows a similar optical image of an LEDproducing blue light, where the device shown comprises an epitaxial ZnOcurrent spreading layer deposited using the current invention onto a(10-11) oriented LED fabricated on a single crystal GaN substrate. Thisdevice produced 106 mW of light at 100 mA of applied current as measuredby an integrating sphere detector. In Ref [3], the use of the ZnO layerimproved the external quantum efficiency of the device by 93% comparedto the industry standard thin NiO/Au current spreading layer. However,it is likely that this can be improved further by incorporating lightextraction structures into the ZnO layer. Such structures can easily beproduced using aqueous processing of ZnO films, by either bottom upgrowth or top down etching.

ZnO Film Properties

The electrical properties of ZnO will be important for manyapplications. The carrier type, concentration, and mobility of epitaxialZnO films fabricated using the present invention have been measuredusing van der Pauw method Hall effect measurements. The films measuredwere not intentionally doped. Like most unintentionally doped ZnO, thefilms display n-type conductivity behavior. Carrier concentrations of asdeposited films produced with the current invention were in the10¹⁸-10¹⁹ range. The most likely source of electron donors, i.e., then-type dopant, is hydrogen incorporated into the ZnO films duringsynthesis. The theoretical role of hydrogen in ZnO is described in Ref[5]. Doping with hydrogen supplied by the aqueous growth solution,allows ZnO films with relatively high carrier concentrations to bysynthesized without intentionally doping with a column IIIb element likeB, Al, Ga, or In. The electron mobility in as deposited films hastypically been measured to be in the 25-50 cm²/Vs range. These electronmobility values roughly correspond to the range seen in vapor depositedfilms, and achieve about 25-50% of the electron mobility observed inbulk single crystals with roughly the same carrier concentration,according to Ref [6], indicating that the ZnO films are of good quality.

Another important property for applications where the ZnO acts as atransparent conductor, such as when integrated onto a III-N LED asdescribed above, is transparency to visible light. An epitaxial ZnO filmfabricated with the current invention with a thickness of approximately5 μm was measured to transmit greater than 92% of the total light in therange of 390 nm to 700 nm, which roughly corresponds to wavelengthsvisible to the human eye. The same film was measured to absorb between94% and 95% of the ultra violet light with wavelengths between 300 nmand 375 nm. The sharp onset of absorption below 375 nm corresponds to adirect optical band gap of 3.31 eV, which is the value typicallyreported for hexagonal wurtzite structure crystalline ZnO. The highlevel of absorption at wavelengths below the band gap and hightransparency above the band gap indicates a high quality film. Forexample, the films created with the present invention transmit morevisible light than the undoped, or Ga doped ZnO films produced by pulsedlaser deposition in Ref. [7].

Process Steps

FIG. 1 illustrates a method of fabricating ZnO. The method may comprisethe following steps.

1. The use of an aqueous solution composition to dissolve 124 a zinccontaining nutrient at a first temperature. The zinc containing nutrientmay be ZnO. The aqueous solution may contain complexing ligands, have anappropriate pH, and otherwise comprise a composition where zinccomplexes may form which result in higher solubility at a first, cooler,temperature compared to a second, warmer, temperature. The complexingligand used may be ammonia so that zinc ammine complexes form insolution. The nutrient may be ZnO that has been doped or alloyed withanother substance.

2. Moving, or allowing to move 100, the aqueous solution between atleast two zones including the first zone 102 at the first, cooler,temperature containing the nutrient and a second zone 104 at the second,warmer, temperature wherein ZnO is synthesized, so that a net flux ofZnO occurs from the first zone 102 to the second zone 104. However,during one or more process steps the aqueous growth solution may beprevented from moving between the first 102 and second 104 zone.

3. One or more process steps may comprise changing the respectivetemperatures of the first 102 and/or second 104 zones by heating 110and/or cooling 118 the first 102 or second zones 104. The solution inthe second zone 104 may be heated 110 to obtain a temperature in thesecond zone 104 that is warmer than the temperature in the first zone102. The solution in the first zone 102 may be cooled 118 to obtain atemperature in the first zone 102 which is cooler than the temperaturein the second zone 104. The solution in the first zone 102 may be cooled118 and the solution in the second zone 104 may be heated 110 to obtaina temperature in the second zone 104 that is warmer than the temperaturein the first zone 102. Both the first, cooler, temperature, at which thenutrient is dissolved 124, and the second, warmer, temperature, at whichZnO is synthesized 116, may be between the freezing and boiling point ofthe aqueous solution composition used.

4. Synthesis 116 of the ZnO at a second temperature (e.g., using theaqueous solution composition), where the second temperature at which ZnOis synthesized 116 is higher or warmer than the first temperature atwhich the nutrient was dissolved 124, and wherein the synthesis 116 ofZnO is caused by a reduction in the solubility 112 of ZnO in the aqueoussolution at the second, warmer, temperature compared to the first,cooler, temperature. The rate of ZnO synthesis 116 may be controlled byvarying any combination of aqueous growth solution composition, thetemperatures of the first 102 and second 104 zones, the heating 110and/or cooling 118 rates of the first 102 and second 104 zones, and therate at which the solution moves 100 between the first and second zones.The nucleation 116 rate of ZnO, either on a substrate or template or asfree nuclei, may be controlled by varying any combination of solutioncomposition, the temperatures of the first 102 and second 104 zones, theheating 110 and/or cooling 118 rates of the first 102 and second 104zones, and the rate at which the solution moves 100 between the first102 and second 104 zones.

The ZnO synthesized 116 may be doped or alloyed with other components.For example, the growth solution may contain dissolved species which areincorporated into the ZnO that is synthesized. These dissolved speciesinclude, but are not limited to, ions containing elements which areincorporated into ZnO as dopants.

ZnO synthesis 116 may result in the formation of a ZnO film on asubstrate in contact with the aqueous solution 302 b. For example, thefilm 400 may be in contact with a MgAl₂O₄ substrate 402. The substratemay have already been seeded with ZnO. The resulting ZnO film may beepitaxial with the substrate. The substrate may be a III-N based LEDdevice, for example, as shown in FIG. 5( a) and FIG. 5( b).

ZnO synthesis 116 may result in the formation of ZnO microstructures ornanostructures 404, as shown in FIG. 4( c). The microstructures ornanostructures can form either on or within a substrate template or formin the bulk of the solution.

ZnO synthesis 116 may result in growth of a bulk ZnO single crystal.

The aqueous solution may contain additives which affect the synthesis116 of ZnO but are not consumed during ZnO synthesis 116 and are notincorporated into the resulting ZnO. These additives could include, butare not limited to, metal citrate salts, citric acid, other salts oracids which produce stable anions which interact with the surfaces ofZnO, surfactants, polymers, and biomolecules.

The above process steps may be performed in a continuous circulationreactor for fabricating ZnO, comprising a dissolution zone 102 or vessel304, for dissolving a ZnO nutrient into an aqueous growth solution 302a; and a deposition zone 104 or vessel 308, for depositing ZnO from thegrowth solution 302 b onto a substrate or seed; and a connection 312,320 to the dissolution zone 102, 304 such that movement of the growthsolution 302 a, 302 b from the dissolution zone 102 to the depositionzone 104, and vice versa, occurs. The dissolution zone 102 and thedeposition zone 104 may be for operation at one or more temperaturesbetween the freezing and boiling points of the aqueous growth solution302 a, 302 b.

As shown in FIG. 3, the dissolution zone 102 and the deposition zone 104may be contained in one or more vessels 304, 308 which may be fabricatedfrom nonreactive materials that will resist corrosion, dissolution, orotherwise be degraded by contact with the aqueous growth solution 302 a,302 b. Such materials include, but are not limited to, fluoropolymers,other higher performance polymers, silicate glass, and stainless steel.All components of the reactor which come into contact with the aqueousgrowth solution 302 a, 302 b during operation may be comprised orcomposed of the nonreactive materials.

The reactor may further comprise a peristaltic pump 310, tube pump, orother mechanical pumping mechanism capable operating in a manner thatdoes not contaminate the aqueous growth solution 302 a, 302 b beingpumped, positioned to pump the growth solution from the dissolution zone102 to the deposition zone 104, to control the circulation 100 rate ofthe growth solution 302 a between the dissolution zone 102 and thedeposition zone 104.

The reactor may further comprise a filter 316 positioned between thedissolution zone 102 and the deposition zone to prevent any, or limitthe particle size of, undissolved nutrient being pumped from thedissolution zone 102 to the deposition zone 104.

The reactor may further comprise an overflow mechanism 320 positioned toreturn flow 108 of the growth solution 302 b from the deposition zone104 to the dissolution zone 102, and for ensuring that a volume of thegrowth solution 302 a, 302 b in each zone 102, 104 remains constantwhile avoiding the need for a second synchronized pump to return thegrowth solution 302 b to the dissolution zone 102.

The reactor may further comprise a heater 322 in thermal contact withthe deposition zone 104; and a temperature sensor 324 in thermal contactwith the deposition zone 104, wherein the heater 322 is for heating thegrowth solution 302 b in the deposition zone 104, such that the growthsolution 302 b in the deposition zone 104 is a warmer growth solutionand the growth solution 302 a in the dissolution zone 102 is a coolergrowth solution that is cooler than the warmer growth solution, and thetemperature sensor 324 enables measurement and control of thetemperature in the deposition zone 104 through an electronic temperaturecontroller or computer connected to both the heater 322 and thetemperature sensor 324. However, in one modification, the solution 302 bin the deposition zone 104 of the reactor may be heated by absorption ofradiation, for example by absorption of 2.45 GHz microwave radiation,rather than by thermal contact with a heater 322.

In another possible modification, the substrate or seed crystal in thedeposition zone 104 of the reactor is first heated and heat is thentransferred from the substrate or seed to the surrounding aqueoussolution 302 b.

REFERENCES

The following references are incorporated by reference herein.

[1] “Controlling Low Temperature Aqueous Synthesis of ZnO: Part 1,Thermodynamic Analysis,” by Jacob J. Richardson and Frederick F. Lange,Cryst. Growth Des. 2009, 9(6), pp. 2570-2575.

[2] “Controlling Low Temperature Aqueous Synthesis of ZnO: Part II, ANovel Continuous Circulation Reactor,” by Jacob J. Richardson andFrederick F. Lange, Cryst. Growth Des. 2009, 9(6), pp. 2576-2581.

[3] “Light Emitting Diodes with ZnO Current Spreading Layers Depositedfrom a Low Temperature Aqueous Solution,” by Daniel B. Thompson, JacobJ. Richardson, Steven P. Denbaars, and Frederick F. Lange, AppliedPhysics Express 2 (2009), 042101.

[4] “Low Temperature Aqueous Deposition Of ZnO on GaN LEDs,” by JacobRichardson, Daniel Thompson, Ingrid Koslow, Jun Seok Ha, Steve DenBaars,and Fred Lange, presentation slides at SSLEC annual review, November2009.

[5] “Hydrogen as a Cause of Doping in Zinc Oxide,” by Chris G. Van deWalle, Physical Review Letters (2000), 85(5), pp. 1012-1015.

[6] “Resistivity of Polycrystalline Zinc Oxide Films: Current Status andPhysical Limit”, by K. Elmmer, Journal of Physics. D (2001), 34(21), pp.3097-3108.

[7] Cheung, Jeffery T., “Transparent and Conductive Zinc Oxide Film withLow Growth Temperature”, U.S. Pat. No. 6,458,673 B1, 2002.

CONCLUSION

This concludes the description of the preferred embodiment of thepresent invention. The foregoing description of one or more embodimentsof the invention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. Many modifications andvariations are possible in light of the above teaching. It is intendedthat the scope of the invention be limited not by this detaileddescription, but rather by the claims appended hereto.

What is claimed is:
 1. A continuous circulation reactor for fabricatingZnO, comprising: a dissolution zone, for dissolving a ZnO nutrient intoan aqueous solution composition when the aqueous solution compositionincludes one or more sources of complexing ligands; a deposition zone,for depositing ZnO from the aqueous solution composition onto asubstrate; and one or more physical connections between the dissolutionzone and the deposition zone, capable of allowing movement of theaqueous solution composition from the dissolution zone to the depositionzone and/or from the deposition zone to the dissolution zone and whereinthe complexing ligands have a greater impact on ZnO solubility in thedissolution zone than in the deposition zone.
 2. The reactor of claim 1,wherein the dissolution zone and the deposition zone are contained inone or more vessels which are fabricated from nonreactive materials thatresist reaction, corrosion, dissolution, or other degradation by contactwith the aqueous growth solution.
 3. The reactor of claim 2, wherein thenonreactive materials include one or more of the following:fluoropolymers, other higher performance polymers, silicate glass, orstainless steel.
 4. The reactor of claim 2, wherein all components ofthe reactor which come into contact with the aqueous solution duringoperation are comprised of the nonreactive materials.
 5. The reactor ofclaim 1, further comprising at least one pumping mechanism capable ofmoving the aqueous solution composition from the dissolution zone to thedeposition zone and/or from the deposition zone to the dissolution zone.6. The reactor of claim 1, further comprising at least one filter orseparation mechanism capable of preventing any, or limiting or selectingthe particle size of, undissolved nutrient in the aqueous solutioncomposition being transferred from the dissolution zone to the synthesisor deposition zone.
 7. The reactor of claim 1, further comprising: atleast one heating mechanism capable of heating the deposition zoneand/or the dissolution zone; and/or at least one cooling mechanismcapable of cooling the deposition zone and/or the dissolution zone. 8.The reactor of claim 7, wherein the at least one heating mechanismprovides radiation that either directly or indirectly results in heatingof the aqueous solution composition.
 9. The reactor of claim 8, whereinthe radiation is 2.45 GHz microwave radiation.
 10. The reactor of claim7, wherein the at least one heating mechanism provides heat or heatinducing radiation to the substrate in the reactor and the heat of thesubstrate is subsequently transferred from the substrate to thesurrounding aqueous solution composition.
 11. The reactor of claim 1,further comprising at least one mechanism for selectively opening orclosing at least one of the physical connections to either controllablyprevent or allow the movement of aqueous solution composition betweenthe dissolution zone and the deposition zone.
 12. The reactor of claim11, further comprising at least one temperature sensor capable ofdirectly or indirectly measuring a temperature of the deposition zoneand/or the dissolution zone and transmitting that temperatureinformation to an electronic temperature controller or computer capableof controlling the operation of at least one heating mechanism orcooling mechanism of the reactor.